The aerodynamic performance of aerodynamically lifting surfaces or airfoils, such as wings, is displayed to the pilot of an aircraft for safety reasons. A critical performance parameter of the airfoil surfaces is stall margin.
Stall detection or warning devices are almost universally fitted to aircraft. The role of the stall warning device is to augment, or substitute for, the natural stall symptoms which may vary according to the aircraft configuration, weight, attitude, and the maneuver being performed. Aircraft can rely on stall detection devices which fall into three groups: those actuated by a hinged vane mounted on the leading edge of the wing, sensitive to the position of the stagnation point of the airflow; angle of attack systems (AOA); and airfoil aerodynamic performance monitors (AP).
Wing leading edge devices sense the movement of the stagnation point as it transitions from above the vane to below it, as the stall is approached. These systems have disadvantages which limit their effectiveness. Among these disadvantages are the fact that the vane is prone to interference from gusts and turbulence G loading, only a limited number of vanes can be fitted on an airfoil, and the vanes are prone to icing.
Angle of attack is defined as the angle formed between the wing chord line and the direction of flight. At any specific angle of attack the airflow over some percentage of the wing surface will generate lift as well as some amount of drag. Maximum lift of the airfoil is usually obtained at a relatively high angle. However, if this angle is further increased, by even a small amount, the airflow over the wing becomes disturbed and buffeting may be felt. Stall is defined as the condition which arises when the angle grows so large that the flow is completely disrupted and not enough lift is generated to overcome the weight of the aircraft. This angle of attack, the stall angle, is constant for a particular airfoil (although various airfoil designs stall at differing angles).
The amount of useful lift and drag generated by any airfoil at some specific angle of attack will depend upon the influence of such variables as the airfoil geometry, density altitude, aircraft gross weight, velocity, etc. However, the ratio of lift to drag coefficients at a given angle remains constant. Therefore, the theoretically ideal ratio of lift and draft coefficients for any flight maneuver will always be found at the same angle of attack under all speed or load conditions.
Although angle of attack is a valuable reference measurement for realizing optimum performance during climb, cruise or landing, contamination on the airfoil affects the stall angle such that the theoretical stall angle based on airfoil geometry differs from the actual stall angle. AOA systems are calibrated when the aircraft wings are free of contamination so the stalling point of the aircraft can be linked to the attitude of the fuselage of the aircraft. This method of stall warning is relatively simple and accurate as long as the wing remains free of contamination. When contamination is present, however, the lifting and stall characteristics of an airfoil change, and the aircraft stalls at a lower angle of attack. In this case a conventional stall warning system with a fuselage mounted sensor no longer provides an accurate measure of the actual stall condition of the aircraft.
Aerodynamically monitoring the performance of an aircraft lifting surface provides this missing component in stall warning, which is the ability to measure the premature loss of lift due to contamination (such as insect deposits, snow, slush, or ice) on the lifting surface. Conventional stall warning systems which use a fuselage mounted angle of attack sensor do not measure the actual stalling condition at the wing. The key to determining an early stall due to the presence of contamination is to measure the flow directly at the lifting surface. Local velocity changes in the region above the upper surface of the wing provide a consistent indication of an approaching aerodynamic stall even when contamination is present.
U.S. Pat. No. 4,435,695 entitled "Method Of Predicting The Approaching Stall Of An Aircraft Wing" to John M. Maris discloses a method of predicting the approaching stall of an aircraft wing by utilizing a probe which measures the local steady state and turbulent components of the airflow at a predetermined location on the top surface of the wing. When the ratio of turbulent flow to steady state flow exceeds a threshold a signal is produced which indicates that a stall is imminent. Contamination on the wing is therefore factored into the stall margin determination in order to provide real time stall warning data.
Referring now to FIG. 1, a typical prior art cockpit display 2 indicates a singular stall margin derived from either an AOA sensor or an AP sensor. A needle 4 indicates a value or range from 0 to 1.0, where 1.0 on the scale represents a stall. A pilot determined how much of a margin there was between the current sensor reading and the sensor reading at which the aircraft will stall.
Referring now to FIG. 2, an alternative display 6 for indicating AOA or airfoil aerodynamic performance was a bar graph type presentation.
If the aircraft was equipped with an AOA detection system the pilot could determine the margin between actual AOA and the theoretical stall AOA.
If the aircraft was equipped with an AP detection system the pilot could determine the margin between actual aerodynamic performance and the actual stall.
Efforts to improve stall warning systems have led to continuing developments to improve their reliability, versatility, practicality and efficiency.